1. Field of the Invention
The present invention relates generally to a process for producing an aryl carbamate from an aromatic polyamine with a diaryl carbonate and to a process for producing aryl isocynates and polyureas from the obtained diaryl carbamates.
2. Description of the Related Art
Organic isocyanates have widespread industrial applications. The isocyanates manufactured in the largest volume are the organic di- and polyarylisocyanates employed in polymer manufacture, particularly in the production of polyurethanes, polyurethane/ureas, polyisocyanurates, and related polymers.
Both aryl as well as aliphatic polyisocyanates are useful. Aryl diisocyanates such as 2,4- and 2,6-toluene diisocyanate (TDI) and 4,4′-diphenylmethane diisocyanate (MDI) dominate isocyanate production because of cost and performance considerations, their reactivity profiles, and their utility in polyurethane molded and slabstock foam productions. However, the production process of MDI and TDI, which have enjoyed an annual gross production of over 3.5 million tons globally, are still being practiced by the phosgene process. Because of toxicity of phosgene used and corrosive hydrogen chloride generated in the production, green and non-phosgene processes of producing MDI and TDI has been intensely sought for the last thirty years to better comply with public demand and work-place safety.
In the developments of non-phosgene processes for MDI and TDI, the most sought-after substitutes of phosgene in the research have been urea/alcohols, di-substituted carbonates and carbon monoxide as the carbonylation reagents for making biscarbamates as the precursors to isocyanates. For example, Olin discloses a process of converting nitro groups into isocyanates with carbon monoxide in the presence of platinum and rhodium as catalysts, but the process has not been accepted by the industry, because of the low yield of isocyanates and harsh reaction conditions of high temperature and pressure, and expensive catalysts recovery problems.
Essentially since Olin's process, most if not all of the non-phosgene routes involve a carbonylation step to convert diamine- or dinitro-groups into their corresponding biscarbamates. Then, MDI and TDI are generated by thermolysis of the respective biscarbamates to liberate the isocyanate groups from the alcohols.
For example, U.S. Pat. No. 3,895,094 (ARCO process) discloses a process for the manufacture of a methyl phenyl carbamate by reacting a nitrobenzene, methanol, and carbon monoxide in the presence of a catalyst of selenium, which is then reacted with formaldehyde under acidic condensation to form N,N′-dimethyl-4,4′-methylenediphenylene-biscarbamate. N,N′-dimethyl-4,4′-methylenediphenylene-biscarbamate is then subjected to thermolysis to form 4,4′-methylenediphenylene diisocyanate (MDI). Nevertheless, the catalyst, selenium, applied in this process is toxic and therefore is not good to the environment; and the temperature for the thermolysis process is very high (240 to 260° C.). In addition, the Se catalyst is difficult to be re-cycled and there are many by-products produced during the thermolysis process. Therefore, this process has not been accepted in industry.
Later in 1978, U.S. Pat. No. 4,547,322 (Asahi process) modified the ARCO process and used palladium (Pd) and sodium iodide as the catalysts instead. This process used aniline, carbon monoxide, and ethanol as reactants to form ethyl phenyl carbamates, which was then reacted with formaldehyde under acidic catalyzed condensation to form N,N′-diethyl-4,4′-methylenediphenylene biscarbamate. N,N′-diethyl-4,4′-methylenediphenylene biscarbamates was then subjected to thermolysis to form 4,4′-methylenediphenylene diisocyanate. Nevertheless, the catalyst, palladium, applied in this carbonylation process is very expensive and difficult to recycle. Moreover, the thermolysis temperature is still high (about 250° C.). Therefore, this process is not economically attractive.
Given the disadvantages described above, bis-N,N′-dimethyl-4,4′-methylenediphenylene biscarbamates (4,4′-DM-MDC) or N,N′-diethyl-4,4′-methylenediphenylene-biscarbamate (4,4′-DE-MDC) have not been implemented practically for MDI production, though they were touted as the most important precursors for non-phosgene synthesis of MDI since 1980s.
Yamazaki's study showed a carbonylation reaction of 4,4′-methylenedianiline (4,4′-MDA) with diphenyl carbonate (DPC) producing 4,4′-DP-MDC in a yield of 68% with 2-hydroxypyridine as the catalyst (J. Polymer. Sci. Polym. Chem. 1979, 17, 835). More recently, Harada (U.S. Pat. No. 6,143,917) found that the yield of N,N′-diphenyl-4,4′-methylenediphenylene biscarbamate (4,4′-DP-MDC) could be enhanced by treating 4,4′-MDA with a diphenyl carbonate in the presence of carboxylic acids such as pivalic acid or benzoic acid as the catalyst at a temperature of −30° C. to 200° C. The carbonylation reaction scheme of MDA is as follows.

Under later conditions, high conversion (>90%) with high purity of 4,4′-DP-MDC was found achievable in toluene.
4,4′-DP-MDC produced above can be then subjected to thermolysis to form 4,4′-MDI. At present, the thermolysis process can be carried out in a gaseous state or liquid state. The gaseous reaction is usually performed at a temperature of 400 to 600° C. in the presence of a Lewis acid as catalyst, and the liquid state reaction is usually performed at a temperature of 80 to 300° C. Since the gaseous reaction requires very high processing temperature and the yield of 4,4′-MDI is low, the liquid state process is more frequently used in undergoing the thermolysis process to form 4,4′-MDI.
Polyurea and polyurea elastomers have been known to possess many outstanding mechanical properties. Currently, important polyurea commercial markets are in automotive and construction applications with familiar finished products such as bumpers, fascia, waterproofing linings, thermal insulation materials, industrial flooring and sports facilities. However, due to high reaction rates of diisocyanates such as methylenediphenylene diisocyanates (MDI) and diamines, the synthesis and processing of polyurea and polyurea elastomers have been heavily dependent upon the assistance of reaction injection molding (RIM)-machine or high-pressure mixing equipments. Synthesis of polyureas in bulk through more controllable step-wise manners has rarely been reported and will be highly prized for polyurea formulation and products development. If the synthesis of polyurea can completely avoid using diisocyanates such as MDI or TDI as the raw materials, it would become even more attractive because of elimination of highly toxic and reactive diisocyanates in the overall synthetic scheme.
Therefore, in a non-isocyanate route (NIR) to polyurea, the approach based on MDI-biscarbamates seems promising because no high-temperature condition seems required through trans-esterification or trans-ureation. In fact, several attempts have tried to use carbamates, or biscarbamates directly as precursors for polyurethane (PU) and polyurea synthesis bypassing isolation of diisocyanate completely (J. Polym. Sci. Polym. Chem. 1979, 17, 835). A model rapid and selective trans-ureation reaction scheme of synthesis of 4,4′-diphenylmethanbis-[(2-hydroxyethyl)urea] has been achieved in dimethyl sulfoxide (DMSO) solution as exemplified as follows where the urea derivative was formed in a high selective yield. However, no report of trans-ureation polymerization of N,N′-diphenyl-4,4′-methylenediphenylene biscarbamate (4,4′-DP-MDC) has been reported under DMSO or tetramethylene sulfone (TMS) prior to the study.

Metal or Lewis acid catalyzed trans-esterifications of diols and diamines with biscarbamates also have been studied in recent reports with some successes (J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 2351) but the use of metal catalysts in the process is neither economically nor environmentally acceptable. In addition, the molecular weights of PUs made through those trans-esterification processes are generally not high.
Recently, other non-isocyanate synthesis of making polyurea and polyurethanes has been explored with reaction of diamines with new intermediates. For example, Meijer uses di-tert-butyl tricarbonate and diamine terminated poly(tetrahydrofuran) serving as raw materials for making thermoplastic elastomers (TPEs) of urea segmented block copolymers (Macromolecules 2005, 38, 3176; Macromolecules 2006, 39, 772). Another approach for preparing polyureas without isocyanate chemistry was reported in Mtilhaupt's studies where a carbonyl biscaprolactam served as a non-halogen building block that could convert the terminal amine-groups of functional polymers into the corresponding caprolactam-blocked isocyanates (Angew. Chem. 2003, 42, 5094; Macromolecules 2003, 36, 4727). Although these approaches appear to be non-isocyanate processes to polyureas in nature, high cost of di-tert-butyl tricarbonate and carbonyl biscaprolactam seems limiting their wide-spread applications.